Preparation of sucrose monoesters



2,999,858 PREPARATIQN F SUCROSE MONOESTERS Gerard Warren Curtis, Fail-lawn, N.J., assignor to Robert S. Aries, New York, N.Y. N0 Drawing. Filed Sept. 18, 1956, Ser. No. 610,640 14 Claims. (Cl. 260-234) This invention relates to improved methods for making monoesters of sucrose, and more specifically to a process for preparing a monoester reaction product of sucrose with a hydrophobic, monocarboxylic acid.

Monoesters of sucrose, in which one hydroxyl group of sucrose is esten'fied, have been found to be particularly eflicient as detergents, surface-active agents, and sequestering agents. Broadly speaking, these surface-active compounds depend upon the presence of a hydrophobic radical and a hydrop-hilic radical combined in the same molecule. In the sucrose monoesters, the sucrose is the hydrophilic radical, and the hydrophobic radical comprises any of the acidic, hydrophobic radicals capable of entering into an esterification reaction with sucrose. Many such acidic, hydrophobic radicals are now used in soaps and synthetic detergents, and these are useful in forming suitable surface-active monoesters of sucrose. Among these acidic, hydrophobic radicals are those of the naturally occurring fatty acids, both saturated and unsaturated; the hydroxy acids such as ricinoleic acid and the hydroxystearic acids; the substituted aliphatic acids, such as the naphthenic acids, chaulmoogric acid, or similar synthetic acids such as phenylstearic acid; branchedchain aliphatic acids such as those obtained from the 0x0 process; rosin acids and combinations of rosin and fatty acids such as tall oil and hydrogenated tall oil acids; synthetic aliphatic acids obtained from the oxidation of petroleum or paraiiin wax; and the odd-numbered synthetic aliphatic acids such as margaric acid.

The lower alkyl esters of the naturally occurring higher fatty acids having from 12 to 18 carbon atoms exclusive of the ester group have been found particularly suitable for forming sucrose monoesters. By lower alkyl esters, I refer to those esters having from one to five carbon atoms. The term fatty acids as used herein is intended to embrace the aliphatic monocarboxylic acids. This includes not only the straight-chain, unsubstituted saturated acids, but also the unsaturated and branched-chain acids. By higher fatty acids are included those acids from caprylic acid upward, including unsaturated acids, such as oleic acid, and through behenic acid. Particularly included are the C to C acids. Those fatty acid esters I find particularly desirable for the practice of this invention are the methyl, ethyl, and propyl esters of lauric, myristic, palmitic, and stearic acids.

Heretofore, it has been considered essential, and the practice has been, to dissolve sucrose and the methyl ester of a fatty acid in the presence of an inorganic basic catalyst such as potassium carbonate in a solvent capable of keeping all of the reactants in solution. Furthermore, in order to increase the yield of the sucrose monoester at the expense of higher esterification products, the molar ratio of sucrose to fatty acid ester has been maintained at about 3:1. In one such process heretofore used, a solution of sucrose and the methyl ester of a fatty acid have been heated together with a small amount of potassium carbonate in a relatively large volume of dimethyl formamide under vacuum at 90-95 C. for about 12 hours. Dimethyl formamide has been used because it is one of the few solvents that will keep all of the reactants in solution.

The use of dimethyl formamide as a reaction solvent possesses several disadvantages which renders it undesirable from the standpoint of cost of production of the desired products on a commercial scale. Thus on a per pound basis, the cost of dimethyl formamide is much rates fiatent 2,999,858 Patented Sept. 12, 1961 ice operation of the process. Conducting the reaction under vacuum, which has been the necessary practice heretofore, makes a high percentage recovery of solvent more difficult than were the reaction to be run at atmospheric pressure. Furthermore, if an attempt is made to reduce the time cycle of the reaction by running the reaction at temperatures above C., undesired by-products result. Thus where a steanic acid ester is used, the method is rendered economically unfeasible because of the large amounts of dimethyl stearamide formed. This reduces the yield of the desired monoester and results in eXcessively high losses of the solvent dimethyl formamide.

It is an object of the present invention to provide an improved method, particularly suitable for use on a commercial basis, for producing sucrose monoesters.

It is a further object to provide an inexpensive hydrocarbon medium for carrying out the sucrose esterification reaction.

It is an additional object to reduce the cost of the sucrose esterification reaction by lowering the molar ratio of sucrose to fatty acid ester below that heretofore con-. sidered feasible.

It is still an additional object to provide an esterification reaction between sucrose and a fatty acid ester wherein the fatty acid ester serves as a suspension medium for the sucrose.

It is still an additional object to provide a method for preparing sucrose in a finely divided state particularly suitable for use in the esterification reaction of the subject invention.

It is a feature of this invention that the requirement for the monoesterification of sucrose of a solvent capable of keeping all the reactants in solution has been eliminated. I have discovered that it is not necmsaiy to keep all the reactant ingredients in a homogeneous solution in order to attain a high rate of reaction. Thereby much cheaper reaction media in which sucrose and the basic catalyst compound have a very low solubility, i.e., they are substantially insoluble therein, may be used. Suitable reaction media are the aromatic or aliphatic hydrocarbons or mixtures thereof. As a particular feature of this invention, the sucrose and the basic compound, such as one selected from the alkali and alkaline-earth oxides, hydroxides and carbonates, are reacted in suspension in a finely divided state with a lower alkyl ester of a monocarboxylic acid, and particularly of a naturally occurring fatty acid having from 12 to 18 carbon atoms in its chain, exclusive of the ester carbons. While the fatty acid ester is ordinarily soluble in the hydrocarbon medium, the sucrose and basic catalytic compound exist as finely divided suspensions therein. It is preferred that the sucrose and the basic compound be in as finely divided a state as feasible. While particle sizes up to 200 microns in diameter are considered feasible for use in this process, it is particularly preferred that the particle size used be below 1 micron, and preferably of colloidal dimensions.

I have further discovered that the use of small amounts of amines such as tr-i-n-butylamine in conjuction with the catalyst such as potassium carbonate has been found to facilitate the reaction. The amount of amine required is preferably less than 2% by weight of the reaction mixture. Tertiary aliphatic amines that do not distill out of the reaction mixture while the alcohol is being fractionated out should be used for optimum results. Tri-npropylamine and any of the isomeric tri-butylamines or tri-amylamines have suitable boiling points.

Because of the elimination of the requirement for a solvent'in which the sucrose as well as the fatty acid is soluble, many hydrocarbon solvents nonreactive with the fatty acid ester may be employed in the practice of this invention. Basically, the hydrocarbon solvent used functions to dissolve the monocarboxylic acid ester and to act as a suspending agent for the sucrose and the basic compound. Any hydrocarbon solvents having a suitable boiling range may be employed. It is preferred that the initial boiling point of the hydrocarbon reaction medium should be above 100 C. and below about 160 0., this distillation range being determined in accordance with American Society of Testing Materials (ASTM) method D86-54, which appears on pp. 7-13 of ASTM Standards, 1955 edition. Various aliphatic and aromatic hydrocarbons and mixtures thereof may be used having the desired inertness and boiling range. Thus a hydrocarbon fraction of petroleum origin having a boiling range of about 115 to 135 C. is satisfactory. Particularly suitable is a mixture of the isomeric xylenes having a boiling range of about 136-145 C.

One advantage of using the isomeric xylenes or a similarly suitable hydrocarbon medium is that these compounds are much less expensive than the dimethyl formamide used in the solvent process. Another advantage is that because of their inertness, these hydrocarbons do not participate in side reactions at reflux temperature. Thereby, practically complete conversion of the methyl and other esters of the naturally occurring C to C fatty acids to sucrose esters can be obtained in a relatively short time by using temperatures of from about 120 C. to 150 C. When dimethyl formamide is used as the reaction medium in this temperature range, the yield of desired product is much lower and substantial amounts of valuable raw materials are lost because of the formation of side products, such as dimethyl stearamide.

By reducing the reaction cycle, because of the higher temperatures that can be used, the rate of output of product per dollar of investment in equipment is greater than has been achieved by any method heretofore employed. This renders this method particularly suitable for commercial large-scale purposes. A further important advantage of this method lies in the fact that equally good results can be obtained by this method by using a 2:1 or even lower molar ratio of sucrose to lower alkyl ester of fatty acid or monocarboxylic acid. With dimethyl formamide, as practiced heretofore, it is necessary to use a 3:1 molar ratio or sucrose to alkyl ester of fatty acid in order to obtain a high content of the sucrose monoester in the product. Attempt to reduce the sucrose to fatty acid ratio using dimethyl formamide as the reaction medium results in excessive amounts of the unwanted diesters of sucrose in the product. By using a 2:1 or lower molar ratio, as practiced in the present invention, there is less unreacted sucrose present to be recovered in subsequent operations.

In the general practice of this invention, finely divided sucrose and the finely divided basic compound, one selected from the alkali and alkaline-earth oxides, hydroxides, and carbonates, such as potassium carbonate, are first added to the reaction medium, which consists of a suitable liquid hydrocarbon having an ASTM distillation range between 100 and 160 C. The mixture is heated to reflux temperature with good agitation, and a lower alkyl ester, such as the methyl ester, of the fatty acid is then added at reflux temperature and at atmospheric pressure. The resulting mixture is heated and stirred at reflux temperature, and the methanol formed in the reaction is simultaneously removed by means of a fractionating column. Conversion of the methyl ester of the fatty acid to the mono fatty acid ester of sucrose is substantially complete within about 5 hours at reflux temperature.

The reaction mixture is then cooled and filtered, preferably by means of a centrifuge. The filter cake which contains unreacted sucrose and mono fatty acid ester of sucrose is next stirred with three and one half times its weight of S to sodium chloride solution. The lower aqueous layer is then drawn ofl and the product layer is dried under vacuum. In this manner mono fatty acid esters of sucrose are obtained in yields of -96% based on the amount of methyl ester of fatty acid charged to the reactor. The sucrose mono fatty acid ester content of the product ranges from 92 to 98% depending on the fatty acid ester used.

Without being restricted thereto, in order to more clearly illustrate the practice of this invention, the following examples are set forth in greater detail:

Example 1 A quantity of 228 grams of finely divided sucrose and 6 grams of finely divided potassium carbonate was suspended in 800 grams of a mixture of isomeric xylenes having an ASTM distillation range of l36-145 C. Thereby an approximately 20% slurry of sucrose in the reaction mixture was provided. The mixture was heated at reflux temperature with rapid agitation. To this, 99 grams of methyl stearate was then added. The reaction mixture was then refluxed and stirred for 5 hours. The refluxing was done in a fractionating column which removed the methanol formed in the reaction. The reaction mixture was cooled and the solids were separated by filtration. Unreacted sucrose was then removed from the solids by adding them to 1500 grams of an 8% aqueous soltion of sodium chloride in which the product was insoluble. The aqueous phase was drawn off and the product was then dried under vacuum. The yield of product which contained 94% sucrose monostearate was of theoretical based on the weight of methyl stearate charged to the reaction.

Example 2 An amount of 342 grams of finely divided sucrose and 5 grams of finely divided potassium carbonate was suspended in 2180 grams of a mixture of hydrocarbons of petroleum origin having an ASTM distillation range of to C. The mixture was heated at reflux temperature with rapid agitation. To this, 100 grams of methyl laurate was then added. The mixture was stirred and refluxed at 130 C. for 5 hours while removing the methanol formed in the reaction by means of a flactionating column. The reaction mixture was then cooled and the solids were separated by filtration. The product, which was isolated by the method employed in Example 1, contained 93% by weight of sucrose monolaurate. The yield of product was 94% of theoretical based on the weight of methyl laurate charged to the reaction.

Example 3 An amount of 228 grams of finely divided sucrose and 6 grams finely divided potassium carbonate was suspended in 1420 parts by weight of hydrocarbons of petroleum origin having a distillation range of 110 to C. The mixture was heated at reflux temperature with rapid agitation, and 104 grams of ethyl stearate was added. The reaction mixture was heated under reflux for six hours while simultaneously fractionating out the ethanol formed in the reaction. The reaction mixture was then cooled and the solids were separated by filtration. Unreacted sucrose was then removed by stirring the filter cake with 1500 grams of an aqueous solution of sodium chloride. The product was then separated and dried under vacuum. The yield of product containing 95% by Weight of sucrose monostearate was 94% of theoretical based on the weight of ethyl stearate charged to the reaction.

Example 4 the reflux condenser. In order to obtain a fine dispersion of sucrose it is essential to add the sucrose solution at a rate that is considerably lower than the rate at which the water is vaporized out of the mixture. The sucrose solution may be added as a slow stream from above the surface of the hydrocarbon medium or it may be introduced underneath the boiling hydrocarbon in the form of fine droplets by means of spray nozzles.

While this invention has been described with respect to specific process steps and specific examples of reactants, these-are illustrative and non-limiting, and it is believed obvious that various changes and modifications may be made without departing from the nature, scope or spirit thereof, and this invention is not restricted thereto except as set forth in the appended claims.

I claim:

1. The process of preparing a monoester reaction product of sucrose and a higher fatty acid in which an alcohol by-product is formed, comprising reacting finely divided sucrose with a lower alkyl ester of a higher fatty acid in the presence of a finely divided basic compound selected from the group consisting of the alkali and alkaline-earth oxides, hydroxides and carbonates in a liquid hydrocarbon medium in which sucrose is substantially insoluble, removing the alcohol lay-product d formed, and recovering the sucrose monoester in substantial yield as principal reaction product.

2. The process according to claim 1 in which said finely divided sucrose and said finely divided basic compound are first suspended in said liquid hydrocarbon medium, and said higher fatty acid ester is then added thereto.

3. The process according to claim 1 wherein the reaction is carried out at a temperature between 110 and 160 C. at substantially atmospheric pressure.

4. The process according to claim 1 wherein the sucrose is reacted with the methyl ester of a fatty acid selected from the group consisting of lauric, myristic, palmitic and stearic acids in a molar ratio of sucrose to fatty acid ester of not more than 2 to l.

5. The process according to claim 1 wherein the hydrocarbon medium is selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons, and mixtures thereof having a distillation range between 110 and 160 C.

6. The process according to claim 1 wherein the liquid hydrocarbon medium comprises a mixture of isomeric xylenes having a distillation range between 135 and 145 C.

7. The process of preparing a monoester reaction product of sucrose and a higher fatty acid in which an alcohol by-product is formed, comprising reacting finely divided sucrose at a reaction temperature between 110 and 160 C. with a lower alkyl ester of a higher fatty acid in the presence of a finely divided basic compound selected from the group co g of the alkali and alkaline earth oxides, hydroxides and carbonates, and further in the presence of a tertiary aliphatic amine substantially nonvolatile at the reaction temperature, said reaction occurring in a liquid hydrocarbon medium in which sucrose is substantially insoluble, removing the alcohol by-product formed, and recovering the sucrose monoester in substantial yield as principal reaction product.

8. The process according to claim 7 in which the amine is selected from the group consisting of tri-n-propylamme, tri-butylamines and tri-amylarnines.

9 process according to claim 8 in which said amine 1s present in an amount of less than 2 percent by weight of the total reaction mixture.

10. In a process for preparing in substantial yield a monoester reaction product of sucrose and a higher fatty acid in which finely divided sucrose is reacted with a lower alkyl ester of a higher fatty acid in the presence of a finely divided basic compound selected from the group consisfing of the alkali and alkaline earth oxides, hydroxides and carbonates in a hydrocarbon medium in which sucrose is substantially insoluble, the step of preparing finely divided sucrose for use in said process, comprising adding an aqueous sucrose solution to the surface of the hydrocarbon medium at a rate less than the rate at which water is vaporized from said solution.

11. -A process according to claim 10 wherein the temperature of the hydrocarbon medium is maintained between and C.

12. In a process for preparing in substantial yield a monoester reaction product of sucrose and a higher fatty acid in which finely divided sucrose is reacted with a lower alkyl ester of a higher fatty acid in the presence of a finely divided basic compound selected from the group consisting of the alkali and alkaline earth oxides, hydroxides and carbonates in a hydrocarbon medium in which sucrose is substantially insoluble, the step of preparing finely divided sucrose for use in said process, comprising introducing a concentrated sucrose solution in the form of a fine spray below the surface of said hydrocarbon medium maintained at substantially its boiling temperature.

13. The process of preparing a monoester reaction product of sucrose and a monocarboxylic acid in which an alcohol by-product is formed, comprising reacting finely divided sucrose with a lower alkyl ester of a hydrophobic monocarboxylic acid in the presence of a finely divided basic compound selected from the group consisting of the alkali and alkaline-earth oxides, hydroxides and carbonates in a liquid hydrocarbon medium in which sucrose is substantially insoluble, removing the alcohol by-product formed and recovering the sucrose monoester in substantial yield as principal reaction product.

14. The process according to claim 13 wherein the molar ratio of the sucrose to not more than 2 to 1, and the liquid hydrocarbon medium is selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons and mixtures thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,013,034 Cox et a1. Sept. 3, 1935 2,589,226 Carson Mar. 18, 1952 2,831,854- Tucker et a1. Apr. 22, 1958 2,831,855 Martin, Apr. 22, 1958 2,893,990 Hass et al. July 7, 1959 OTHER REFERENCES Markley: Fatty Acids, Interscience Publishers, Inc., New York (1947), pp. 292-293.

the monocarboxylic acid is a 

1. THE PROCESS OF PREPARING A MONOESTER REACTION PRODUCT OF SUCROSE AND A HIGHER FATTY ACID IN WHICH AN ALCOHOL BY-PRODUCT IS FORMED, COMPRISING REACTING FINELY DIVIDED SUCROSE WITH A LOWER ALKYL ESTER OF A HIGHER FATTY ACID IN THE PRESENCE OF A FINELY DIVIDED BASIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE ALKALI AND ALKALINE-EARTH OXIDES, HYDROXIDES AND CARBONATES IN A LIQUID HYDROCARBON MEDIUM IN WHICH SUCROSE IS SUBSTANTIALLY INSOLUBLE, REMOVING THE ALCOHOL BY-PRODUCT FORMED, AND RECOVERING THE SUCROSE MONOESTER IN SUBSTANTIAL YIELD AS PRINCIPAL REACTION PRODUCT. 